DVB-S and D-ATV

Now that we have taught you the basics of communication theory we are entering the section which discusses DVB-S thoroughly. One big question might arise.. why is D-ATV based on the commercial DVB-S standard? That answer can be explained.
When seeking for new ways to get rid of the 'old fashioned' analog amateur television you'll first start to think about digital implementations. That is a good starting point because a digital system has some serious advantages. Unfortunately there are also some disadvantages. We will start with the advantages and work out why DVB-S is chosen so far for D-ATV.

One of the main advantages of a digital ATV system is the fact that picture quality is improved above that of most analog systems. We do not encounter the negative effects of noise. We do not encounter video group delay problems, an item on which much attention has been paid by lots of amateurs. Audio quality is improved. With digital ATV we get high quality audio channels and these high quality audio channels don't even disturb our picture quality! Also it nearly does not extend our occupied bandwidth of our signal, something which is the case with the old fashioned way where we do need some FM modulated audio carriers above our video signal.
Other main advantages are the fact that analog ATV systems occupy a lot of bandwidth. And a wide occupied bandwidth means several disadvantages:

• Less room for others to communicate,
• Higher noise bandwidth

The first item is clear. We want to be as efficient as possible. If this can be done without throwing away any quality then this is nice. If we even can improve quality with less occupied bandwidth then we have even more profit!
The second item is also very interesting. The higher the bandwidth of a signal the higher the received noise level will be at receiver side because noise is integrated over bandwidth. At the same time modulation schemes are characterized by their minimum threshold levels, which determine the ability to demodulate the modulated signal at low signal to noise ratios. Some digital modulation schemes are able to demodulate at lower threshold levels if we compare this situation to the 'old fashioned' FM ATV systems. One of them is for example QPSK. Now we have two main advantages over our second item. We are able to occupy less bandwidth and we can make use of lower thresholds. This means that generally spoken we could get more out of such a system with less power, better quality and less bandwidth!

Another advantage of a digital ATV system is the fact that bit errors or bursts of bit errors due to imperfections in our transmission path can be corrected. In the past decennia a lot of new coding techniques have been developed which makes digital communication a very robust system.

Al these advantages have already been discovered several years ago by the commercial broadcasters. For digital television transmission systems this has leaded to the development of common standardized transmission modes. Hands and brains have been put together and that is how DVB came into life (http://www.dvb.org).
The DVB organization developed three main standards for the transmission of digital television signals. These differences where needed because the transmission media differed on some specific points.

• Transmission from satellite to earth and earth to satellite. This has been developed as the DVB-S(atellite) standard.
• Transmission on cable systems. This had been developed as the DVB-C(able) standard.
• Transmission for Terrestrial. This has been developed as the DVB-T(errestrial) standard.

All three transmission media have some different behaviour. This is the reason why these three different standards have been developed. For example, the satellite to earth transmission path will be characterized by lots of signal path attenuation and line of sight communication. Therefore such a system needs low threshold demodulation. Signal to noise ratio will be worse and therefore only QPSK can be used. QPSK is a very robust modulation scheme as seen before because it just has to make a decision in one of four quadrants. The low signal to noise ratio on the other hand will be a source for bit errors, both burst errors as single bit errors. To overcome this weakness, the DVB-S standard uses different layers of Forward Error Correction (FEC) for a very robust protection against any kind of errors. The FEC consists of a Reed Solomon coding which protects against burst errors and also an additional convolutional interleaving to spread out the impact of burst errors. Beside that the system also takes any measures against bit errors by means of convolutional encoding. The convolutional encoding is better known among users of satellite television (although they might not know that....) and is recognizable in a satellite receiver setup menu under the menu item FEC rate. The fact that satellite communication will result in line of sight communication without to worry about obstacles which are placed between the transmission path tells us that less attention is paid in this system on multipath effects. Therefore, the DVB-S standard will be moderate when it comes to robustness against multipath reflections.
The DVB-C standard is developed for digital Television transmission on cable systems. A cable environment is a relative protected environment with respect to distortion and signal path attenuation. Higher signal to noise ratios can be achieved and the fact that there is no negative effect of multipath this standard is able to implement higher order modulation schemes. These modulation schemes are mostly restricted starting from QPSK up to 256QAM. Under very good defined environments this is extended up to 1024QAM! The FEC implementation for DVB-C is weaker than the case for DVB-S because less environmental disturbances exist. The FEC is limited to the use of a Reed Solomon encoder and convolutional interleaver for protection against burst errors. DVB-C generally requires higher signal to noise ratios at receiver side due to the higher order modulation schemes and the weaker FEC implementation. This is one reason why DVB-C is not preferred above DVB-S for Digital Amateur Television. Besides that, DVB-C is due to its higher order modulation schemes more susceptible to multipath reflections then DVB-S. When we compare to hardware related issues then we see that a lot of commercial chipsets are available for DVB-C. Therefore there will be no need to build it from scratch in FPGA hardware. Besides that, when an implementation is done in FPGA hardware then generally spoken this will require more FPGA area than for a DVB-S modulator implementation. This is due to the fact that the symbol shaping filters will require larger multipliers because the higher order modulation scheme requires a larger word length at the input of the filter.
Finally we get to the DVB-T standard. This standard was developed for terrestrial communication with the aim to overcome the destructive effects of multipath reflections. The datarates for broadcasting services are high. Therefore, the higher the bitrate the higher the negative effects of multipath reflections. The path attenuations can be frequency dependent and as a result from that this can result in a partly distorted received signal. Also the multipath reflections cause Inter Symbol Interference because reflections of the received signal interfere with the direct received path. It should be clear that the higher the bitrate or symbolrate, the higher the negative effects of these disturbances.
With terrestrial communications there will be a big chance on multipath due to the fact that mostly no line of sight communication exists due to all kind of obstacles. There is a way to overcome these disturbances. With DVB-T the effective bitrate is spreaded out over a large amount of digital modulated carriers. These different carriers are generally modulated with QPSK or QAM constellations. The larger the amount of carriers, the lower the effective bitrate that can be used for every single carrier. The lower the effective bitrate per carrier, the lower the negative effects of multipath reflections will be.
This is the basical idea behind DVB-T. Spreading out the bitrate over a large amount of carriers. But now we come to the point, how do we create such a large amount of digital modulated carriers? For DVB-T this will be 1705 carriers for the 2K mode and 6817 carriers for the 8K mode. You can imagine that it will be impossible to make such a amount of different frequency synthesizers with VCO's and PLL chips. Furthermore, another very important issue is the fact that all these different carriers have to be spaced from each other in such a way that they do not interfere with each other. In difficult terms this is called 'orthogonality'. The carriers must be orthogonal spaced.
There exist a mathematical way to create all these carriers orthogonal spaced from each other. This is done with the Inverse Fast Fourier Transform also called IFFT. Now it works as following: The incoming bitstream is encoded with Forward Error Correction blocks like Reed Solomon and convolutional interleaving and finally convolutional encoding. After the FEC the resulting bitstream is mapped on all the constellations for the separate carriers. The resulting constellations are the input for the IFFT processor block which performs the actual transformation from frequency to time domain. After the IFFT a cyclic extension is performed on the resulting OFDM symbol which is used for the guard interval. The guard interval gives additional protection against multipath reflections. The resultant complex output of the IFFT block can then be converted to RF with an I/Q modulator. As you can see this is a very global description of the most difficult implementation of DVB. Also with the above simple description the name of the modulation scheme is explained; Orthogonal Frequency Division Multiplexing (OFDM).
Although DVB-T is designed for best protection and robustness, it takes a lot of very fast hardware for an actual implementation. Specifically the IFFT block has a big impact on hardware implementation. Beside that, OFDM needs a high signal to noise ratio for demodulation.

If we look at the possibilities for D-ATV then we come to the conclusion that DVB-T will be the ultimate if it comes to robustness. However, the high signal to noise ratio which is needed for demodulation, the big impact on hardware implementation and the fact that commercial DVB-T set-top boxes are not widely available yet, let us come to the conclusion that DVB-T is currently far away for amateur use.
DVB-C has worse error protection, and the higher order modulation schemes result in higher signal to noise ratios needed at receiver side and worse protection against multipath. Also the lack of available commercial set-top boxes at this moment is a reason why this standard is not preferred for D-ATV. If we look to hardware requirements for a transmitter implementation then we have the possibility to use a wide range of commercial chipsets.
DVB-S finally, has a big error protection, uses very robust QPSK for modulation which requires low signal to noise ratios for proper demodulation. It isn't the best choice against multipath. However, the fact that lot of experiments in Germany and The Netherlands ended with very positive results showed out that these negative effects are less badly then expected. Beside that, a lot of cheap commercial set-top boxes exist which is a major advantage for D-ATV use. Finally, hardware implementation is a little bit more difficult then for the DVB-C case but far easier then a full DVB-T implementation.
Conclusion: DVB-S is the best choice so far for D-ATV.
Now that we have described the main differences of the various DVB standards and also named some advantages above the 'old fashioned' analogue television broadcasting techniques we come to the discussion of a big disadvantage.
As we have seen above in the section about non-linear amplification of digital modulated signals, the M-QAM techniques and also OFDM will require very linear amplifiers. With linear we don't talk about as linear as we need for SSB techniques but even more! The large amplitude swings of the carrier introduce very high intermodulation levels when the signal is non-linearly amplified. The effects are seen as spectral regrowth as described above. Although QPSK is quite robust and will still work correctly with quite high spectral regrowth levels, there is also a need to transmit a nicely shaped spectrum in order to be spectrally efficient. As stated before, D-ATV generally will need less power compared to FM TV techniques but this will not mean that the amplifiers need to be smaller! In fact, in order to keep spectral regrowth levels low enough, power amplifiers will need to be biased in class-A and the output drive levels will need to be in the order of 7-10 dB below the 1dB compression point to keep spectral regrowth below -40 dBc. Therefore, a lot of commonly used handy class AB power modules can be thrown away and we have to build our own very linear amplifiers again.